Jumping in with Both Feet on a 20 Seafari

[SailorChlud]

One other thing - the 17p prop has a good bit more cupping on the trailing edge than the 15p - could that make a big difference in sternlift?

[Bushwacker]

Originally Posted by SailorChlud

It is the typical analog tach, not digital, unfortunately, but it is fresh from the dealer last month. Hope it is close to accurate.

First new analog tach I bought from the dealer for my old 115 read high by about 300 rpm, leading me to buy the wrong prop! At least they gave me another tach after they tested the first one. I'd have dealer check your tach accuracy before you buy a prop!

Originally Posted by SailorChlud

One other thing - the 17p prop has a good bit more cupping on the trailing edge than the 15p - could that make a big difference in sternlift?

I think trailing cup is typical of a bow lifting prop that's normally used for max WOT speed. I believe a stern lifter will have a lot of cup in the tip, although any 4B prop will give extra stern lift.

[SailorChlud]

Today I managed a good run in the bay- 12 nm or so. Using the aluminum
3 blade 15x17p prop we recorded WOT top speed of just 30 knots at 5200 rpm, so this seems to be the right prop for the boat so far. Seemed to cruise nicely with 6 people (our typical max load) and a full tank of fuel. That was with the bimini down.

However, at one point we were running in about 3-4 feet of water (Barnegat Bay is shallow) and we hit something submerged with the prop while at low speed, and put a good 1/4 inch ding in two places on the prop. Razzafrats! No damage to the skeg, though, so not sure what we hit.

[Dogbomb]

Congrats on a good day!
I guess the good thing about aluminum is that it's easy to straighten...

[Bushwacker]

Originally Posted by SailorChlud

Today I managed a good run in the bay- 12 nm or so. Using the aluminum
3 blade 15x17p prop we recorded WOT top speed of just 30 knots at 5200 rpm, so this seems to be the right prop for the boat so far. Seemed to cruise nicely with 6 people (our typical max load) and a full tank of fuel. That was with the bimini down. . .

I respectfully disagree. Not sure of optimum rpm for your motor, how accurate your analog tach is, and if you've raised motor enough to get AV plate above water at cruise, but I think you should see at least 5500 rpm at WOT. (I believe optimum on the V-4 T-TECs, which use the same basic block, crank, rods, etc., is 5500-5750.) I ran a 13 3/4 x 15P SST prop on my old carb'd 115 which allowed it to spin up to about 5500 rpm with a very heavy Bahamas-type load and the top raised, and about 5700/30 kts with a light load and top down. I put well over 1000 hrs on that motor with no problems. When you lug down the motor by running a prop with too much pitch, it increases pressure and temperature in the cylinder, increasing loads on the bearings and rings and raising piston temperature that weakens the piston and makes it more prone to detonation or pre-ignition caused by a hot carbon deposit!

You can't hear detonation in a 2-stroke motor because there are no valves to rattle, which is what you hear in a 4-stroke motor that's "pinging", and is caused shock waves from fuel mixture which is exploding instead of burning! In the worst case scenario, the first sign of detonation in a 2-stroke may be when it blows a hole in the top of the piston! The lower speeds on an over-propped motor also reduce exhaust scavenging, leading to more carbon deposits on the piston, rings, heads and exhaust ports. Because the 2-strokes have nearly frictionless needle bearings, very short strokes and very rigid short cranks (good for about 9000 rpm on a V-4!), high rpm doesn't hurt 'em, and the harder you run 'em the more oil they get, so you're much better off to err on the high rpm side when selecting a prop!

Next time you're out, I would also test with bimini top up, which will further reduce your WOT rpm, so you may actually need an increase of about 400 rpm. That would typically require a pitch reduction of about 2", so when you get that dinged prop repaired, I would ask the prop shop if they could reduce pitch to about 15". You may not lose that much speed because the prop will be spinning faster with the motor higher up on it's HP curve, plus it will accelerate and climb on plane quicker. Once you get the aluminum prop dialed in, you'll then have a good baseline for selecting a more expensive stainless prop if you want a bit more speed and efficiency! I picked up about 3 mph with the heavily cupped SS prop because it allowed me to raise motor a couple holes.

[SailorChlud]

Denny: My manual indicates a WOT rpm range of 4500-5500, and based on recommendations from several sources, I was under the impression that a WOT rpm figure in the top 50% of the range (5000-5500) was a good target. Is that not the case? My Evinrude dealer/mechanic advised to shoot for close to 5000 rpms.

My first test with bimini up and only 3 people aboard was approximately equal to the collective load on my second test with 6 people and no bimini. I think the loads should be similar, no?

[Bushwacker]

Until BRP published the Optimum RPM ranges for the E-TEC 's, which is a much narrower range than the "Recommended Operating Range", the latter range is the only thing most dealers ever talked about! However the dealer I used for my old 115 had been selling outboards since the 1930's and seemed to be very knowledgeable. They insisted that the motor would be much happier and live longer if I propped it to turn 5500 (the top of the "Recommended Operating Range") with the max load, and that's what I'd do if had your motor. I believe Fr. Frank would say the same thing.

If they repitch your prop to 15" and rpm is too high with max load, a good shop should be able to add some cup in the tip that would bring rpm down and also add some stern lift. That would help lower your min planing speed which you will like because of how it improves the ride if you ever get into a rough chop.

[SailorChlud]

On the outing before this past one, a 15 pitch prop (Michigan Wheel I think) turned around 5700-5750 on the last outing. I thought that was not advised because it was above the 5500 max recommended?

I have another 15 pitch prop (BRP I believe) that I will try next. It seems to be a little bit different (thicker blades).

[Bushwacker]

Originally Posted by SailorChlud

. . . I have another 15 pitch prop (BRP I believe) that I will try next. It seems to be a little bit different (thicker blades).

My 14 3/4 x 15P BRP Cyclone 4B SS prop has a very heavily cupped and very thick trailing edge on the blades, so it acts like it has more pitch than 15"! It's actually harder to turn than my larger 4B SS 15 x 15P PowerTech; the Cyclone turns about 100-150 rpm less @ WOT than the PT, but it's 2-3 mph faster! If the BRP prop is a 3B SS Rebel, it may be very hard to turn as it has very large blades and lots of cup. My dealer said they're normally only used on the big block 3.3-3.4L V-6 motors.